Why would you expect FoundRange to "re-find" firstFind?  Haven't you
replaced the 3600 with something else?

The problem is compounded by how VB(A) examines compound boolean
expressions.  It insists on evaluating all elements even if the final
decision can be reached with only a partial examination.  So, in the Loop
Until x OR y construct, VB(A) will try and evaluate both X and Y.  
Unfortunately, if X is true, Y will fail!

You may have to develop a somewhat more cumbersome mechanism to exit the
loop.  Or, use
FindAll
http://www.tushar-mehta.com/excel/tips/findall.html

If you still want to roll your own, use something along the lines of the
untested

   Set FoundRange = Nothing
   Set FoundRange = SearchRange.Find("3600")
   If Not (FoundRange Is Nothing) Then
       firstFind = FoundRange.Address

       Dim AllDone as boolean 'new/changed
       AllDone=false '

       Do
           If Left(FoundRange.Formula, 1) = "=" Then
               FoundRange.Replace What:="3600", Replacement:="GWhToGJ"
           End If
           Set FoundRange = SearchRange.FindNext(FoundRange)

           AllDone=FoundRange is nothing '
           if not alldone then alldone=foundrange.address=firstFind '
       Loop Until AllDone '
   End If